Libxc ID notation and XClib testing data

• From now on the ID notation only will be allowed for xc functionals that include Libxc terms (see the user_guide or the comments inside). In this way the dft name management is easier to maintain and there is no risk of overlaps among different dft names (especially between Libxc and non-Libxc ones);
• the possibility to use mGGA exchange + non-mGGA correlation or vice versa has been added, with a warning message;
• two reference xml files have been added for the XClib test (@ye-luo : I have set the Cmake dependency for the test and it seems to work, but feel free to modify it if you think).